Piezo-electric device and method of producing it



May 4 ,1926. 1,583,417

I A. M. NICOLSQN PIEZO ELECTRIC DEVICE AND METHOD (DP PRODUCING IT FiledFeb. 4, 1925 hue/rial": A/examerM Mao/son Patented May 4, 1926. I

qumrnn STAT ALEXANDER M. NICOLSON,

MENTS, T0 WESTERN NEW YORK.

IPIEZO-ELECTRIC DEVICE AND METHOD, OF PRODUCING IT.

Application filed February 4, 1925. Serial. No. 6,862.

To allwhom it may concern:

Be it known that I, ALEXANDER M. Nioon SON, a subject of the King ofGreat Britain, residing at New York, in the county ofNew York and Stateof New York, have invented certain new and useful Improvements inPiezo-Electric Devices and Methodsof Producing Them, of which thefollowing is a full, clear, concise and exact description.

This invention relates to vices and method of producing the same, andaims to provide sensitive, eflicient, simple and durable devices of thatcharacter, for translating electrical Variations into mechanical forcevariations, and vice versa, such devices being useful, for instance, inthe transmission and reception of telephone currents and thephonographic recording and reproduction of sound waves.

In accordance with the invention a piezoelectric crystal, preferably aRochelle salt crystal having the composite or hour-glass structure andhaving orthogonal electric poles, such as is described in my article onThepiezo-electric effect in the composite Rochelle salt crystal,published in the Proceedings of the American Institute of ElectricalEngineers, November, 1919, is provided with electrodes or electrically'conducting, charge collecting means in contact with the parts of thecrystal which are oppositely electrified when the crystal is stressed,one of the charge collecting means being preferably in intimate contactwith the interior of the crystal. For example, the means for collectingthe charges from the pyramidal poles or hour-glass shaped portions ofthe crystal may be an electrical conductor 0r conductors within orpassing through those portions and cutting various imaginary planesperpendicular to the principal axis of the crystal, that is, planesparallel toand of the crystal, as shown in my copending application,Serial No. 655,800, filed August 16, 1923.

At the place where. this interior electrode or the lead for connectingit in circuit passes through the surface of the'crystal it may occupy avery small portion of that surface, and therefore it is easy to insurethat the leakage resistance along the sur-.

face of the crystal from this interior electrode to the chargecollecting conductor of piezo-electric delying between the basal planesopposite sign will be maintained high, as for instance, by bringing theinterior conductor, or its lead, through the surface of the crystal at apoint distant from the conductor of opposite sign, or by bringing theinterior conductor or its lead through the surface in an ,insulatingtube or covering, or by employing both of these features.

Further, by reason of' the fact that this electrode is within thecrystal, intimate contact may be maintained between the electrode andthe crystal without any necessity of employing means to make theelectrode integral with the crystal.

A better understanding of the invention will be had from the followingdetailed description in connection with the accompanying drawings inwhich:

Fig. 1 is a side elevation of a loud speaking telephone receiverembodying one form of the invention;

i Fig. 2 is a front elevation of the structure of Fig. 1; a

Fig. 3. is a cross-sectional view in side elevation along the line 3-3of. Fig. 2;

Fig. 4. is a perspective of a piezo-electrie crystal of Rochelle saltshowing the axes of. the crystal;

Fig. 5 is a front elevation of a crystal having an interior electrode ofa different.

form from that shown in Fig. 1;

Fig. 6 is a side elevation of Fig. 5.

Referring especially to Figs. 1 and 2, a piezo-electric crystal 10having the. composite structure referred to above is mounted n a base11. The crystal may be produce by methods such as, for instance, thosedescribed in the above mentioned A. I. E. E. Patents No. 1,414,370, May2, 1922, method of making piezo-electrical crystals, and No. 1,438,965,December 19, devices and method of producing the same, or in mycopending application Serial No. 756,390, filed December 17. 192A.

Attached to the upper end or upper basal plane of the crystal is an arm.12 which carries a diaphragm 13 to be in response to twisting of thecrystal about its principal, or vertical axis. The crystal may be causedto twist by applying anelec-v tromotive force to girdle electrode, 14and OF NEW YORK, N. Y., 'ASSIGNOR, BY MESNE ASSIGN- ELECTRIC COMPANY,INCORPORATED, A CORPORATION OF publication, in my 1922, piezo-electrievibrated by arm 12 1 an electrode which is imbeddedin the crystal.Conversely, '-.e1 ectromot1ve" forcemay be generated between theseelectrodes by twisting the crystal, as for instance by causing soundwaves to impinge upon the diapragm 13.

The girdle electrode shown is of the character described in the A. I. E.E publication mentioned above, and also described intheabove mentionedPatent No. 1,438,965.

In aco-rdance with the invention the in ternal electrode 15 preferablycomprises a plate conductor, as a metal sheet for example, lying in aplane cutting approximately the middle of the crystal along the a axisand lying parallel to the I) and 0 axes, and extending substantially thewhole length and width of the crystal so that it will cover imaginaryplanes passed perpendicular to the surface and lying between the apexesof the pyramids and-points near the ends of the crystal. The portion ofthe pyramidal pole lying between any one of these planes and the apex Iof the pyramidal pole may be regarded as a pyramidal pole having thatplane as its basal plane, and electrode 15' passes through and contactswith a great number of such planes thus collecting the electricalcharges appearing on them when the crystal is stressed and conductingsuch charges to the outside of the crystal by means of the lead 16connected to or forming a part of the conductor 15, it being understood,of course, that lead 16 may be connected to one terminal of a receivingapparatus (not shown) the other terminal of which may be connected to alead 17 brought from the girdle electrode 14 of the crystal. When thecrystal device is used as a receiver, the source of electromotive force(not shown) from which it is to be operated is connected between leads16 and 17, and the conductor 15 then acts as a charging means instead ofas a collector of electrical charges.

The electrode 15 comprises in accordance with the invention, a flatconductor of area preferably of the same magnitude as that of the back39, of the crystal. The combination of electrodes covering a largeportion of the area of the crystal along a plane parallel to the b and caxes with the crystal per se have been found to form articularlyefficient structures for translating electrical variations intomechanical force vibrations and vice versa. This electrode is furtherpreferably of some form designed to have intimate contact with theadjoining crystal surfaces.

One method of imbedding the internal electrode 15 in the crystal 10might be to separate the crystal, as by sawing for example, along aplane near the middle of the (1 axis of the crystal and parallel tothe]; and ,0 axes. After the internal electrode elements 18 and 19 havebeen secured in place and the lead elements 22 and 23 secured to theelectrode elements, in the manner. hereinbefore described, the crystalmay be made integral by joining the halves 28 and 29 with a suitableadhesive applied to the edges of the crystal. One cement compositionwhich has been found satisfactory is the Rochelle salt cement disclosedin my copending application, Serial N o. 7 10,? 46, filed September 3.0,1924:.

The thickness and composition of the particular internal electrode orconductor employed may be considerably varied. Examples of electrodeswhich have been found satisfactory are thin sheets of metal foil, alayer of mercury, saturated Rochelle salt solution and graphite. -Actualpractice has shown that thin metal foil or sheets are especiallysatisfactory in that they both make intimate contact with the crystalsurfaces and are in a solid form in contrast to granular and liquidelectrodes. By actual test, it has been found, for example, that leadfoil having a thickness of approximately ten mils forms a satisfactoryform of internal electrode.

When applying sheets of metal foil, it has been found desirable to applyindividual sheets or conductors to each of the separated surfaces of thecrystal. Referring to Fig, 3

of the drawings there are shown sheets of metal foil 18 and 19, composedof lead, for example, in intimatecontact with the internal surfaces 20and 21 of the crystal 10. There is also represented a lead 16 leadingoutside the crystal which is composed of two sets of conductors 22 and23, one set, 22, being connected to plate 18 and the other set, 23,being connected to the other plate 19, so that intimate contact can bemade with the entire internal electrode surfaces.

Each of these metal foil sheets is prcfer. ably supplied with a thincoating of some material which will act as an adhering medium betweenthe crystal and conductor.

A thin coating of wax, such as beeswax, for example, has been foundsatisfactory, but it is evident that other material might be used toreplace the beeswax.

One means of securing the separte elements 22 and 23 of the lead 16might be to insert a mass of low melting point metal, such as Woodsmetal, for example, in a hole 24in the crystal surface and then meltingthe metal globule 25 around the conductor, as around conductor 22, forexample. In this method, it has been found desirable to add additionalsheets of metal foil 26 and'27 to cover each of the conductors at thepoint at which they are sealed or soldered to the crystal.

Another means of securing a connection between a lead and an electrodemight be a mass of pliable elastic material, such as softrubher, coveredwith a pliable electrical conducting material, as lead foil for example,p aced between the individual electrode elements 18 and '19, the lead 16being inserted between the pliable conducting material andthe electrode,and being held in position-by the pressure of said elastic material.

It has also beenfound desirable to apply an insulating material,preferably in the form of a cement, around the element 16 and where ,itpasses through the crystal surface and also on the associated surface ofthe crystal. A suitable cement has been found to be the Rochelle saltcement referred to above, it being applied to the crystal surface, whendesired, in the manner described in my copending application Serial No.655,800, filed August 6, 1923.

Fig. 5 shows a crystal 10 of the same character as the crystal 10 ofFigs. 1 and 3 and having a girdle electrode 14 of the same generalcharacter as the girdle electrode 14 of crystal 10; the crystal 10,however has an internal electrode divided into .four parts, 30, 31, 32and 33, according to the pyramidal poles or hour-glass structure of thecrystal. These pyramidal poles are designated 34; and 35 in Fig. 2, andare indicated in the drawing by dotted lines 36 converging toward theseedling 37 from which the crystal is grown. To each of the electrodeelements '30, 31, 32 and 33 is secured a lead 38, the method of securingsaid lead being one, such as described above.

Any number of these individual elements 30, 31, 32, n'iay'be connectedin series or parallel to form a composite electrode to be joined to oneterminal of a translating device, the other electrode being the girdleelectrode 14:; or the girdle electrode may be dispensed with, byconnecting elements 30 and 31 through their respective leads to form onecomposite electrode and connecting elements 32 and 33 through theirrespective leads to form the other electrode. The action of a crystalhaving twointernal electrodes instead of one internal and one externalor girdle electrode will be more evident from a study of my article inthe Proceedings of the American Institute of Electrical Engineers,November 1919, above referred to. From this article it will be seen thatthe electric poles of the crystal are orthogonal or that the pyramidalpoles 34c and 35 are positive when-the remainder of the crystal isnegative and vice versa. Series or parallel conductors in contact onlywith the pyramidal poles, therefore, may be em.- ployed scribed inrelation to the structure of Figs.

1, 2 and 3 and sealed in a like manner. While specific examples ofinternal elecas one electrode \vhile series or paral lel conductors incontact only wlth the other portions of the crystal subsequently trodestructures and compositions have been represented herein, it will beunderstood that the invention is not limited to these specific examplesdisclosed but it is susceptible of various adaptations and alternativeforms in accordance with the appended claims.

That is claimed is:

1. A piezo-electric crystal having an electrode in its interior, saidelectrode being of a form designed to cover a large portion of the areaof said crystal lying in a plane parallel to the plane defined by the band 0 axes of said crystal.

Q. .A piezo-electric device, comprising a piezo-electric crystal havinga pyramida pole, a girdle pole about said pyramidal pole, and a plateelectrode in the interior of said crystal. v

3. A piezo-electric crystal having a plate electrode embeddedwithin andsurrounded by said crystal.

4. A piezo-electric crystal having a pyramidal pole, and a plateelectrode within said crystal and intersecting said pyramidal pole.

5. A piezo-ele'etric crystal having a plate electrode in its interior,said electrode lying in a plane parallel to the plane formed by the Z)and c axes of said crystal.

6. A piezo-electric crystal having a plate electrode therein, saidelectrode lyingin a plane in the middle'of said crystal along its (1axis and parallel to the plane formed by its b and@ axes.

7. A piezo-electric crystal-having a plate electrode in its interior,said electrode being composed of metal foil.

8. A piezo-electric crystal having a plate electrode in its interiorsaid electrode being coinposed'of metal foil and having a coating ofadhesive material thereon.

9. A piezo-electric crystal having pyram-. idal poles. a plate electrodeimbeddcd in said. crystal and intersecting said pyramidal poles, acavity within said crystal, a mass of electrode inserted between saidelements and means for cementing said elements to form an integralstructure.

11. A piezo-elcctric crystal having pyramidal poles and beingdivided/into two elements along a plane perpendicular to the basalplanes of said pyramidal poles, and an I electrode inserted between saidelements, said electrode comprising metal foil sheets in contact withthe ad aCent faces of said elements.

12. A piezo-electric crystal having pyramidal poles and being dividedinto two elements along a plane perpendicular to the I a f n adhesivematerial in said-cavity, and a con n basal planes of said pyramidalpoles, ,and

i an electrode inserted between said elements,

said electrode comprising two adjacent integral metal foil sheets incontact with the central portions of the adjoining faces of i saidelements and extending substantially to I the edges of said faces.

13. The method of producing a piezoelectric crystal device whichcomprises sawing a crystal in a plane perpendicular to the (1 axis ofsaid crystal, inserting a plate con electric device which comprisessawing a crystal along a plane perpendicular to the (Z axis of saidcrystal to produce two elements, placing a plate electrode in contactwith each of the adjacent faces of said elements, making a cavity ineach of said faces, placing conductors in contact with said electrodes,and extending beyond the length of said crystal sealingsaid conductorsto said crystal in said'cathodes and joining said elements to form acomposite structure.

In Witness whereof, I hereunto subscribe my name this 4th day ofFebruary A. 1)., 1925.

ALEXANDER M. NICQLSON.

